This invention relates to surface-emitting heterojunction laser/LED structures.
The importance of vertical laser emission for optoelectronic integration was recognized by, for example, K. Iga et al, 9th IEEE International Semiconductor Conference, 1984, Rio de Janeiro, Brazil, paper D3, p 52. Vertical laser emission is particularly important in constructing large emitting areas which can be made to have narrow beam angles and high power outputs. Several types of surface-emitting laser are known. SpringThorpe et al. International Electron Devices Meeting, (1977) Washington, D.C., p. 571, disclose a standard double heterostructure cavity transverse to current flow. The cavity is electrically pumped over most of its length and two additional mirrors are used to divert the laser beam towards the device surface. K. Iga et al. Electronics Letters, 19, #13 (1983) p. 457, disclose a surface-emitting laser having a cavity perpendicular to the surface but pumped over a short length of the cavity by a pn junction co-planar with the surface. Ito et al. Electronics Letters 20 #14 (1984) p. 577, elaborate on the Iga structure by elongating the cavity and introducing additional pumping along its length by a diffused homojunction.
In my co-pending patent application Ser. No. 673,644 there is described an alternative surface emitting device. The device has a columnar active region of one direct bandgap semiconductor and a surrounding confining region of a higher bandgap semiconductor. Contacts are made to the active and confining regions and a window is formed in the device in vertical alignment with the active region to permit light emission from the device. The semiconductors are doped to establish a pn junction within a carrier diffusion length of the heterojunction between the active and confining region, the pn junction extending the length of the active region. In use, light is emitted along the axis of the columnar active region in response to current passing radially across the pn junction.
Any of these surface emitting devices can have multilayered reflectors at the top and bottom of the vertically oriented active region. Alternatively the active region itself can be epitaxially grown to provide layers of varying composition and with layer thicknesses such that in operation wavelength dependent distributed feedback is provided. If the feedback occurs within the active region, such devices are termed distributed feedback lasers (DFB), whereas outside the active region they are generally referred to as distributed Bragg reflectors (DBR), see for example, IEEE Spectrum December, 1983, p. 43. Bragg distributed reflectors for surface emitting lasers are described by Ogura et al. "GaAs/Al.sub.x Ga.sub.1-x As Multilayer Reflector for Surface Emitting Laser Diode", Japanese Journal of Applied Physics, volume 22, No. 2, Feb. 1983, pp. L112-L114, while by Ogura et al. "Distributed Feed Back Surface Emitting Laser Diode with Multilayered Heterostructure", Japanese Journal of Applied Physics, Volume 23, No. 7, July 1984, pp. L512-L514 demonstrates the actual application to a laser device.
The Ogura et al. devices have multilayer semiconductors formed in the active region and at opposed ends of the active region. It may be disadvantageous to use distributed feedback in the active region if the layers of semiconductor material do not lend themselves to the application due to growth, technological or wavelength reasons. In addition, the material quality in the active region should be very high and may suffer as a result of multilayer growth. Monomode fiber applications amongst others require single longitudinal mode lasers of very narrow spectral width. A major problem presently is "chirp", the small but signicant variation in wavelength due to changing drive current. The drive current variation is caused by variation of the refractive index of the active region with carrier density. If the wavelength-determining mechanism (DFB) includes the active region, chirp is often apparent. However, if the wavelength-determining mechanism is outside the active region, chirp should be reduced.
In an alternative distributed feedback device according to the invention, a semiconductor laser diode has an active region of direct bandgap semiconductor material, a confining region of higher bandgap semiconductor material adjacent to the active region, the confining region having a series of layers of changing composition and refractive index, the series of layers epitaxially related to one another and to the active region, the layers located at least partly within a region of wave propagation of the laser and functioning in use to provide Bragg distributed optical feedback .